1. Field of the Invention
This invention is directed to the field of electrodynamically actuated switches of the vacuum or hermetically sealed type (e.g., reed switches).
Magnetically operated reed switches are used in contact applications requiring switching devices capable of repetitive operation over a long period of time without replacement or maintenance. One such application is in the contacts of a burglar alarm system which is mounted on a door frame. There the switch is magnetically actuated each time the door is opened. Other typical applications include limit switches for detecting movement in the elements of machines, and in the keyboard mechanisms of automatic business machines.
Reed switches are generally constructed of a pair of aligned contact reeds of a magnetic material, disposed within a sealed glass envelope or similar container. A portion of each of the reeds protrudes from the sealed container as a lead. Preferably the envelope is elongated, with the leads projecting from its ends. These leads allow the switch to be connected in myriad electrical applications.
In one form, the reeds are aligned in the envelope or container so that they are normally not engaged with each other, i.e., the open position. The switch is closed under the impetus of a magnetic force field which encompasses the reeds and causes them to be attracted toward one another. Reed switches of this kind will last almost indefinitely in a switching mechanism, and are good for literally millions of operations without failure or maintenance.
It can be seen that this invention can additionally be applied to connecting other forms of reed switches, e.g., those wherein the reeds are aligned so that the switch is normally in the closed position, as well as in so-called transfer applications, wherein three contact reeds are disposed within the glass envelope, the first of said three contact reeds being transferred by a changing magnetic field from electrical contact with one of the remaining two contact reeds to the second of the remaining reeds, thereby briefly causing an open circuit during the transfer which can be detected. Moreover, the invention is also applicable to the connection of other glass enclosed switches, e.g., mercury switches.
2. Description of the Prior Art
The connection of reed switches and other similarly configured switches requires that electrical contact be established between the conductor leads projecting from the switch envelope and the external circuit to which the switch is connected. However, the switches themselves, because they are small and fragile, are usually first connected to a connector assembly which provides support and protection, as well as a means for making electrical connection to the external circuit. Then, the completed connector-switch combination is inserted where needed in the external circuit.
Sometimes, the switches must be modified for mounting on a particular connector assembly by bending the leads which project from the ends of the glass envelope. This is necessary, for example, when the leads are to be connected to the assembly by wrapping them around internal terminal posts. However, because these leads form part of the magnetic environment of the switch, deformation of the leads affects the operation of the circuit by changing the switch characteristics. For example, bending the leads raises slightly the magnetic field strength required to actuate the switch (the pull-in ampere-turn value). Another modification often necessary to facilitate external circuit connection is shaping the reed switch lead ends into specific formations. This particularized shaping, such as by lead forming or bending, must be done with great care to avoid damaging the delicate seals between the glass envelope and the metal leads. Any weakening of these vacuum seals substantially increases the risk of switch failure under operating electrical load conditions and decreases useful life.
In some instances, neither lead bending nor forming is sufficient to produce the required lead configuration. In this case, additional conductive material, sometimes termed a pigtail, must be welded or soldered onto the leads to permit circuit connection. Often, the leads themselves are welded or soldered directly to terminals in a switch assembly, as shown, for example, in U.S. Pat. Nos. 3,293,578, 3,321,723, and 3,302,143, all incorporated herein by reference. During such welding or soldering, the leads, displaying heat conductive properties different from the glass envelope, expand and contract much more quickly than the envelope. These unbalanced heat effects create internal stress forces which lead to loose or cracked glass-to-metal seals, thus degrading switch performance as noted previously. Moreover, welding and soldering, laborious and expensive manufacturing steps, increase substantially the cost of the switch.
The prior art, particularly U.S. Pat. No. 3,292,123, incorporated herein by reference, displays the use of a screw engaging a nut to establish the electrical connection of a reed switch. This connection results from clamping each lead of the switch, on tightening the screw, between the bolt and a block through which the screw acts. However, in this arrangement, permanent mechanical deformation of the block material contacting the lead usually occurs due to the stress applied by the screw and nut combination. In a plastic block, for example, this phenomenon of permanent deformation resulting from prolonged application of a stress below the elastic limit of the plastic is known as creep or cold flow. Deformation or cold flow results, in time, in a reduction in the contact force between the lead and the metal bolt. The weakened contact thereby creates a risk of loss of electrical contact within the switch. Thus, in applications where long-term reliability is critically important, such as burglar alarms, the adverse effects of deformation or cold flow on switch contact maintenance must be eliminated. Significantly, this deformation is more pronounced in reed switches having leads of circular cross section and small diameter (e.g., a few mils), than in those having flat leads such as described in United States Patent No. 3,292,123.
Prior connectors for reed switches also use sockets or slots for receiving the conductor leads of the switch. Such an arrangement is shown for example in U.S. Pat. No. 3,271,708, incorporated herein by reference. There the connection to the reed switch leads is made through slotted spring clips. Yet, spring clips have proven to be unreliable in continuous use applications, making them particularly unsuited for burglar alarms and other systems where reliability is a paramount factor.
In summary, prior methods for connecting reed switches to external electrical circuits usually involve the steps of bending, forming, welding or soldering of the leads. The connective methods in addition to being laborious and expensive often result in damage to the switch or changes in its operating characteristics. Moreover, those prior methods which avoid these unfavored connective steps produce electrical circuits with the switch which are not suited to applications where high reliability is an important design factor.